This invention relates to a method of interference-tolerant heartbeat measurement, in which method a person's heartbeat signal is measured at a suitable part of the body and transmitted by telemetric transmission from a transmitter to a receiver as pulse data formed by successive measuring pulses, in which data a transmission interval of at least some pulse signals is proportional to the person's measured heartbeat rate.
Known telemetric transmitters of heartbeat measuring devices typically transmit a burst of about 5 kHz each time they detect an ECG signal. A transmitter circuit is constituted by a simple resonance circuit activated by transistor control. At heartbeat measurement, a transmitter unit sends a signal each time the heart beats. A receiver calculates the heartbeat rate on the basis of the difference of time between the successive signals transmitted, which means that the method in principle consists of time interval coding, i.e. that the data to be transmitted is included in the transmission and coded into the time between the pulses.
The known method described above is simple and reliable in conditions free of interference. Single interferences can be filtered off by comparing the obtained pulse value with preceding results: if a new measurement result differs too much from the preceding ones, it is obviously caused by some outer interference coupled to the transmission-reception channel and may thus be eliminated from the measurement results.
However, if wireless transmission of pulse data is performed in an interfered environment, the situation changes essentially. A continuous irregular pulse train may then arrive at the receiver, from which train to pick up the correct pulse signal is a difficult task, often even impossible. Such a situation arises easily when two or several users of pulsimeters are too close to each other.